A hybrid fiber-coaxial (HFC) network technology is an economical and practical integrated digital service broadband network access technology. An HFC generally includes three parts, an optical fiber trunk, a coaxial cable branch, and a user cable distribution network. A program signal from a cable television station is first converted into an optical signal for transmission on the trunk. The optical signal is converted into an electrical signal in a user area, and sent to a user using a coaxial cable after being distributed by a distributor.
FIG. 1 is a schematic diagram of a typical HFC network. As shown in FIG. 1, the HFC network includes the devices and components, such as a network management system, a proactive network maintenance (PNM) server, a cable modem termination system (CMTS), an optical station, a cable modem (CM), a user-side set top box (STB), a personal computer (PC), and components such as a fiber, a coaxial cable, an amplifier, and an attenuator (the components are not shown one by one in the figure). It can be seen that the CMTS is located on a metropolitan area network side and is also referred to as a head end, and the CM is located on a user end.
A transmission characteristic refers to a relationship between an input signal and an output signal when a signal passes through a device or a channel, and is a parameter that reflects transmission quality and performance of the device or the channel. For the HFC network, the transmission characteristic mainly refers to a relationship curve between an attenuation characteristic and a frequency when a signal passes through the network. This relationship is also referred to as an amplitude-frequency characteristic (amplitude and frequency curve) of the signal. The devices, components, and cables in the HFC network have respective transmission characteristics, and a network structure is complex, resulting in different transmission characteristics from users CM to a head end CMTS.
A transmission characteristic from the CM to the head end is widely used in designing and debugging of the HFC network, and in future operation and maintenance. Proper components and an optimal cascading manner need to be selected for installation and layout when the HFC network is designed and debugged for the first time, to ensure similar path losses of all users. An optical device and an amplifier further need to be debugged after the installation, to finally maintain consistency of transmission characteristics of all the users. In an operation and maintenance aspect, as network usage time goes by, characteristics of all components have different levels of changes and distortion (because of aging, water corrosion, cable bent, and the like), finally resulting in distortion of transmission characteristic curves of all the users. For example, fluctuation or unflatness appears, and some users even encounter relatively severe faults. In this case, locations of the faults need to be analyzed by analyzing the transmission characteristic of the network, to perform line adjustment.
To obtain the transmission characteristic from the CM to the head end, a network signal is usually measured and analyzed using a network analyzer or a spectrum analyzer. However, this manner can only be performed when the entire HFC network is in a power-off state, and cannot be performed when the HFC is in a working state. In addition, this method requires that there is no intrusion signal in the network, such as noise interference. An intrusion signal is measured by an instrument if there is the intrusion signal. Consequently, a line characteristic cannot be correctly reflected, resulting in inaccurate measurement and analysis.